Steam generating and superheating unit with recycled gas flow



July 23, 1957 A. E. RAYNOR ET AL 2,800,116

STEAM GENERATING AND SUPERHEATING UNIT WITH RECYCLED G AS FLOW Tlql.

Filed Oct. 20, 1954- l VENTORS Inn/w? RAY/v01? H AND ATTORNEY 5Sheets-Sheet l July 23, 1957 A. E. RAYNOR EI'AL 2,800,116

STEAM GENERATING AND SUPERHEATING UNIT WITH RECYCLED GAS FLOW Filed Oct.20, 1954 5 Sheetsg-Sheet 2 lNV 'ro JE L. 4mm: 5. $2w539 w y H. wmvoATTORNEY y 1957 A. E. RAYNOR EIAL 2,800,116

STEAM GENERATING AND SUPERHEATING UNIT WITH RECYCLED GAS FLOW Filed Oct.20, 1954 5 Sheets-Sheet'S mvzmons 4? THUR E. Aa Y/VOI? w. 1. 0 vvmv 0ATTORNEY y 1957 A. E. RAYNOR ETAL 2,800,116

STEAM GENERATING AND SUPERHEATING UNIT WITH RECYCLED GAS FLOW Filed001;. 20, 1954 5 Sheets-Sheet 4 I i I I I g I i 192 i /24 i 1 l lINVENTOR5 July 23, 1957,.

STEAM GENERATING AND SUPERHEATING UNIT WITH RECYCLED GAS FLOW Filed001-. 20, 1954 A. E. RAYNOR ETAL, 2,800,116

5 Sheets-Sheet 5 Fig.5.

INVENTORS IRTHUR E. R4 Y/VOP WILL ll OWIND BY fi ATTORNEYW STEAMGENERATING AND SUPERHEATING UNIT WITH RE Y L D GA FLOW Arthur E, Raynor,Rockville Centre, N. Y., and Will H. Rowand, Short Hills, N. J.,assignors to The Babcock 9a Wilcox Company, New York, N. Y., acorporation of New Jersey Application October 20, 1954, Serial No.463,418 7 Claims. (01. 122-240) This invention relates to high capacitysteam generating and superheating units. It is more particularlyconcerned with improvements in such units operating at high furnacetemperatures, high steam temperatures, and high steam Pr ssures, andhaving a high ratio of steam generating surface to furnace or furnacechamber volume.

A steam generating and superheating unit exemplifying the inventionincludes a single boiler setting having a twin or duplex furnacearrangement with separately tired furnaces or combustion chambersseparated by a common wall of steam generating tubes. The remainingwalls of the combustion or furnace chambers separated by this wall havetheir boundaries defined by other steam generating tubes connected intothe circulation of the unit. The furnaces, or furnace sections, arefired by fuel burning means preferably disposed adjacent the lower partsof the separate furnace chambers with the gases flowing upwardlytherethrough on opposite sides of the division wall and then over thesurfaces of separate convection sections. One of these sectionspreferably includes a reheater, as well as some economizer andsuperheater surface, while the other includes other superheater andeconomizer surfaces, In a preferred embodiment of the invention thegases exiting from the different gas flow paths join beyond theconvection sections and pass over an air heater dis.- posed at one sideof that part of the setting including the furnace chambers and theconvection sections. This side m be co i e e s a s e co nec he wo si esof the setting along which the opposite sets of fuel burning means arearranged,

The pertinent arrangement of elements promotes better mixing ofrecirculated gases ahead of the superheater elements because of thesmaller throat Width just ahead of he per e tert als mi imiz unbalatlseda temperatures across the unit, and minimizes the amount of uppo in s ere ed o a un t f a c ta n capac ysf rab the Opposite mes? sha b rs;teearated y the divisi n all, ce gases ro independent se s of .0 burn ncy on u aces na c amb r d p nd & h site s d o ach t'the first me t p s'dfurna chambers which ma be n d e s c dar f r ace hamb r u al y There s aP mary.

f coal rn n y nes teha s nto ea h primar furnace chamber.

F r nt f s a h a n nd or nt l of he absorption of the steam generatingfurnace. wall tubes, over a' wide load range, as well as for promotingthe effecting of optimum combustion by operation of the fuel burningmeans at highest optimum temperatures, partially ooled urnac a e a ehdra n fr m os ions, o a posi n n t gas o pa h d ns eam sfthe s e st snction nd are at qs u d a on o were s cted o itions with reference to gasflow. Preferably, for maintaining an optimum temperature of superheatedsteam as the load on the unit decreases, an increasing amount ofrecycled gases are caused to flow into the primary furnace chambers andthence through the secondary chambers and then over the convectionsections to increase the mass flow of gases and thereby relativelyincrease the heat absorption of the convection section. At the highloads, or in the upper part of the load range, the temperature of thegases at the gas entrances of the convection section may be decreased'or held at a permissible value by the introduction of controlledamounts of recycled gases as tempering gases, at positions just ahead ofthe convection sections.

For increasing the elfectiveness of the introduction of tempering gasesjust ahead of the convection sections, the division Wall has some of itstubes (i. e. one-third) bent to the right of the division wall and someof the remainder of the division wall tubes bent oppositely to the leftof that wall to define a tempering gas duct, or tempering gas entrychamber. Some of the bent-out parts of the tubes may have inter-tubeflat studs welded thereto to better form this tempering gas duct. Inthis way the tempering gases are introduced at the opposite sides ofeach furnace section, to promote optimum mixing of the recirculatedgases with the other gases.

The invention will be concisely and clearly set forth in the claimsappended hereto, but for a better understanding of the invention, itsadvantages, and uses, recourse should be had to the followingdescription which refers to the accompanying drawings in which apreferred embodiment of the invention is shown.

Of h ,rawings:

Fig. 1 is mainly a vertical section through a steam generating andsuperheating unit, on the line 1-1 of Fig. 2; 1

Fig. 2 is a vertical section at right angles to the plane of the Fig. 1section and taken on the dual plane section line 22 of Fig. 3;

Fig. 3 is a plan section on the dual level section line -3 f F Fig. 4 isa vertical section on the dual plane section line 4...4 of Fig. 3; and

Fig 5 is a fragmentary vertical section through one of the convectionsections, on the line 5--5 of Fig. 3.

Fig. 1 shows two opposite secondary furnace chambers 10 and 12 separatedby a division wall 14 consisting of wall aligned steam generating tubesextending from the lower header 16 to the upper headers 18 and 29. Theupper part of this division wall also separates the gas passes 22 and 24for the separate convection sections. The gases flowing through thefurnace section 10 originate in a coal burning cyclone 26, having steamgenerating tubes in its boundaries, and otherwise of a type indicated inthe U. s. patent to Bailey et al. 2,357,301. There be three of thesecyclones, including the other cyclones 2,8

chamber 34 and then upwardly along the right hand wall 1 40 of tsecondary fu a e hamb r The m be further extended 'alongtthe right handwall 42 of the gas (v pass 24 to the header 44 which is appropriatelyconnected to the steam and water drum 46.

At the opposite side of the unit there is a similar arrangement ofcyclones 48, 50 and 52 discharging combustion products into the primaryfurnace chamber 54 for flow therefrom across the screen 56, and thenceinto the secondary furnace chamber 12:.

The floor and outer walls of the primary furnace chambers 34 and 54-preferably include steam generating tubes connected at their lower endsto the lower headers 58 and 60, and extending upwardly therefrom forappropriate connection to the drum 46.

Gases flowing upwardly from the secondary furnace chamber pass over thebanks of tubes 62 and 64 constituting a part of the secondary, or hightemperature superheater. The gases then pass in succession over thebanks of tubes 65-69 constituting the primary, or low temperaturesuperheater. Beyond this superheater the gases pass over the bank oftubes 70, constituting the first section of the economizer. Theycontinue past thegas flow control dampers 72 into the breeching orductwork 74 leading to the air heater having two sections 76 and 78,each including a bank of tubes through which the gases flow.

From the other secondary furnace chamber 12'the gases flow upwardlyacross the banks of tubes 82 and 84 constituting the other section ofthe secondary or high temperature superheater, and thence across thebanks of tubes 86-88 constituting the steam reheater. Following thereheater the gases pass over the banks of tubes 90-92 constituting thesecond section of the economizer. From the economizer the gas flowcontinues past the gas flow control dampers 94 to the ductwork component74, and thence to the air heater including the sections 76 and 78.

The separate gas flows from the separate secondary furnace chambers 10and 12 are combined in the ductwork 74 for flow through the air heaters.From the air heaters the gases enter the breechings or ductworkcomponents 100 from which the gases flow to a stack.

Air, to support combustion in the cyclones passes from an appropriateforced draft fan, or fans, to the air inlet ducts 102 and 104 (Fig. 4),thence through connected ductwork 196 and 108, and then through a firstpass 110, as indicated by the arrow 112 across the gas conducting tubesof the section 78 of the air heater, whence they continue then upwardlyas indicated by the arrow 114 to the second pass 116 in which they flowto the right transversely of the lower part of the tubes of the airheater section 78. These parts of these tubes are separated by adivision plate 118 (Fig. 2 and Fig. 4) from the upper parts of the tubesbetween which the air passes in a third pass as indicated by the arrow120. From this third pass the gases pass downwardly through one or moreducts 122 to the air inlets of the respective cyclone coal burners. Theductwork or chamber 124 provides for the turning of the air stream as itemerges from the pass 112, and its entry into the pass 116, and similarductwork or chamber 126 provides for the passage of gases from the pass116 through the last pass 120.

It is to be understood that the arrangement of air heater componentsjust described is duplicated on the other side of the unit, with theducts 128 directing heated air to the different cyclones 26, 28 and 30.

Feedwater is conducted from an appropriate source to the inlet header139 or the first economizer 70. From this economizer the feedwater flowsfrom the outlet header 132 through an appropriate conduit 134 to theinlet header 136 of the second economizer, including the banks of tubes9092. From the outlet header 138 of the second economizer the heatedfeedwater flows through appropriate connections to the steam and Waterdrum 46.

Steam and water mixtures fed into the drum.46 by all of the steamgenerating tubes are subject to steam and water separation (preferablyby cyclone separators of the type shown in the Rowand et al. Patent2,289,970), with the separated steam passing through tubes 14% to theinlet header 142 of the primary superheater. Thence the steam passesthrough the return bend tubes forming the bank of tubes 6569 and to theoutlet header 144. From this outlet header part of the steam passesthrough the tube 145 to the inlet header 146 of a part of the secondarysuperheater, including the banks of tubes 62 and 64. The remainder ofthe steam from the header 144 passes through the tube 150 to the inletheader 152 of the banks of tubes 82 and 84 constituting the other partof the secondary superheater.

From the outlet ends of the serially connected return bend tubesconstituting the banks of tubes 82 and 84, the superheated steam passesthrough the outlet headers 154 and 156, andthence to a point of use.Similarly at the other side of the unit the steam superheated in theserially connected return bend tubes constituting the banks of tubes 62and 64 of the first part of the secondary superheater passes from thosetubes to the outlet headers 158 and 160 whence the steam may pass tojunction with the steam exiting from the headers 154 and 156, and thenpass to a point of use.

Convection superheaters and reheaters such as those shown in thedrawings tend to give too high a steam temperature at loads above theircontrol point loads. They also tend to give too low steam temperature atloads below their control point loads, and inasmuch as it ishighlyimportant that these inherent tendencies be overcome in order thata predetermined superheat temperature and a predetermined reheattemperature may be maintained over a wide load range, the inventionincludes means for compensating for these inherent tendencies.

For eifecting a predetermined superheat temperature recirculated gassystem which withdraws partially cooled combustion gases from a positiondownstream of the economizer and effects the entry of the returned gasesinto the secondary furnace chambers at a position'just ahead of thesecondary superheater. Effective mixing of the returned gases at thisposition with the gases issuing directly from the fuel burning means notonly promotes superheat and reheat control by regulation of the gastemperature and gas mass flow over the convection surfaces, but it alsopermits the operation of the fuel burning means at gas temperatureswhich would otherwise be too high under some conditions of operation,for safe contact with the convection surfaces. This return of partiallycooled combustion gases and the mixing of such gases with the otherfurnace gases at positions just ahead of the convection surfaces may bereferred to as gas tempering. Such returned gas flow for gas temperingpurposes will be usually at a maximum at high load and high rates offuel burning, and a reduction of this tempering effect may be combinedwith a change of the amount of spray attemperation as a load decreases,to be effective in the over-all steam temperature control.

At low loads, or in the lower part of the load range the returnedcombustion gases are caused to enter the primary furnace chambers forflow through and along walls of those chambers, before entering thesecondary furnace This recirculated gas flow has a double ef-' chambers.fect toward maintaining a predetermined steam temperation surfaces, andthe other part of the effect is the reduction of heat transfer to theheat absorbing surfaces of the furnace chambers. I

Another part of the over-all superheat control system involvesseparately regulable sets of dampers 72 and 94, for the separate gasflows through the gas passes leading from the secondary combustionchambers and across the convection surfaces.

Referring to Figs. 4 and 2 of the drawings the partially cooledcombustion gases are withdrawn from a ductwork component 162 in the pathof the gases leading from the outlets of the separate convection gaspasses to the air heater. Communicating with the ductwork component 162s a d 1 a n to the nle 1 f a circulated gas fan, the outlet 168 of whichis connected o th t mpe n ga utle chambe .180 and 1 an i h th l t d asout et c am ers 84 nd 6 by ductwork components '170176. These ductworkcomponents are appropriately dampered as at 193 and 192, and otherwise,in order that recirculated gas ,flow to the chambers 184 and 186 may beshut .ofi or approp iately controlled throughout a desired range, and inorder that a tempering gas flow to the chambers 180 and 182 may be shutoff or appropriately controlled throughout .a desired load range.

For effective entry of tempering gas at a position just ahead of theconvection sections and for effective mixing of the recycled andunrecycled gases the tubes of the division wall 14 are so constructedand arranged to provide an additional tempering gas outlet chamber orduct 200 at a level adjacent to the level of the tempering gas outletchambers 180 and 182. This tempering gas outlet chamber 200 may bedefined by bending some of the division wall tubes 14 to the right outof their wall forming align.- ment, and then returning them to thatalignment, others of the division wall tubes being bent to the left in asimilar manner. The bent out portions of the division wall tubes mayhave inter-tube stud plates welded thereto to sufficiently define theduct or chamber 200, with distributed openings for the exit of gases.This chamber or duct 2,0,9 extends through the wall of the secondaryfurnaee cha n.- ber ng t Observer in Fi and s cehn ct d to a verticalduct 202 comunicating with the ,outlet of the re,- c cu at s gas n tough h d c erk ccmpene ts 1 2- T duct 202 is pp op ia ely slampe ed a204 r the n r f emp in a .flcw to th o tlet duct of chamber 200.

For the flow of tempering gas from the chambers 180 and 182 into thesecondary furnace chambers '12 and 10 some of the wall tubes along theinner sides .of these chambers 180 and 182 are bent out of their wallalign.-

ment to provide openings through which the tempering gas may flow.

The recirculated gases entering the chambers 184 and 186 flow therefromdownwardly-into the primary furnace chambers 34 and 54. This flow maytake place in openings provided between the headers 210 and 212 andadjacent steam generating tube sections 214 and 216 which are integralwith the tube sections forming the primary furnace chamber screens 36and 5.6. The headers 210 and 212 are connected to the lower headers .ordrums 58 and 60, respectively, by steam generating tubes 220 and 222.Some of the tubes 220 and' 222 may :be bent out of their wall formingalignment to provide openings through which the recirculated gases flowinto the primary furnace chambers 34 and 54. The headers 210 and 212 forthese tubes are appropriately connected into the cinculation of theunit, preferably by vdirect connections to the drum 46.

6 a d '1! ate h P m furnace ch rs 4 a d 55- As the load and firing ratedecrease from the rnanirnum, the flow and entry of the tempering gaseswill be decreased and the flow of recirculatedgases through the chambers184 and 186 will be increased to a manirnum at a predetermined low loadvalne.

It is contemplated that the flow of tempering gases to the upper partsof the secondary furnace chambers 10 and 12 shall be automaticallycontrolled from registered and recorded variables, such as gastemperatures at the upper parts of the secondary furnace chambers,representations of load and steam temperatures. Similarly, it iscontemplated that the flow of recirculated gas from the outlet chambers184 and 186 shall be automatically controlled from registeredrepresentations of load and steam temperatures.

Ga fl hroug the a P ss 2. eadin u wardly r the s c d ry fur ace cham e12 and e e the eheater n l i th beaks c t bes 8 y e $942: trolled byoperation .of the dampers 94 at the'ontlet of this gas p The n o .c tese damper as we s the control c t e damp rs .2 a he outlet o the as p24 leading r m he s nda y furnace cham e 19. 10? y m y be su h s o p ent as fl w ve the ehe t r u ing s n up f h unit when o s a s throu h thereheater, for the purpose of protecting the reheater, but may also besuch as to limit-all attemperation to superheated steam only. In thisevent, and when the dampers 72 an 74 a au cmeti ll op r t d eh cf t p emnept in ue ce in such operat o i l be hesteam tea per tu at m adj centhe cht et .ct th reh tere. the Pu p s of periodi al emoving a umulaon ocl s arch the he t absehp w su face in the c daw .fhr ae c ambers 9 and1.2 and in the et n s t eh et eet ble se t lowers are .QBQ 'MQQ hreush pnin s indi a ed t 2s -2s4 in Fi 11 eus t l t bl we s ind cated a 236.haerahle hr ug in in the side a 2. 8 in .F ss- 2 and 5- e pp site l 2 ii dicat d a havin .a s mila arrangement cf .Iet a Iib e sect lowe s 24,operates h r r s Be s o he t in u n ce hst h es o t l t t d uni i h ts disi n el f generating tubes, h r e se h nsehsiqh cf t echvecticn c i andt e s cchdsr utne e chambers across which t .retractiblc sec blowers a eepera et i m e ly reduced, in e de h t sect hle ers e educcd l gths mayb s d: Th inc ease t arai b y c t n incr asin t e lite cf the see! lo eco thei s er e ileh th- The l nge h blowers 'used in the illustrated unithave a length .of the order of 20 feet. Referring to the disclosure of'Fig 2, it will be noted that the air heater including the sections 76and 78 is spaced sufliciently from the wall 226 of the secondary furnacechamberand the convection section to permit the operation of the sootblowers 240 "between the fa ing walls of the air heater and the furnaceand convection section component of the unit.

The lateral spacing of the soot blowers 236 operated through openings inthe wall 228 is such that they do not place any substantial limitationupon the location of downcomers intermediate the length of the steamwater drum 46. The downcomer 250, together with the end downcomers 252and 254 leaddirectly downwardly frornthe drum 46 to the level of thelower headers or drum 16, 58 and 60. The intermediate downcomer 250 isjoined directly to the lower drum 16, and the downcomers 252 and 254 areintegral with the lower drums 6t) and 58, respectively.

The drum 46, with its depending downcomers 250 2 52 and 254, as well asthe remainder of the main components of the unit are pendently supportedfrom the top of steelwork including the uprights 300-408 joined at theirupper ends by horizontals 310-3 17. The beams, or

horizontal elements of the steelwork 317 are preferably disposed inpairs along the top of the 'steelwork and in the direction of the lengthof the drum 46, with the drum straps 320 disposed between the elements317, of each of said pair. In this way, and with the arrangement ofelements shown and described, the drum may be supported at a pluralityof positions distributed lengthwise of the drum, to the end that drumbending stresses are materially reduced, or minimized.

Fig. 2 of the drawings indicates the furnace walls. 226 and 228 as linedby rows of steam generating tubes. The tubes 330, along the wall 228have their lower ends connected to the lower header 332 which ispreferably in appropriate connection with one or more of the lower drumsor headers 16, and 60. The upper ends of these steam generating walltubes are connected to the upper header 334 which, in turn, is connectedto the drum 46 by the tubes 336. At the opposite side of the furnacechambers there is a similar arrangement of tubes and headers includingthe lower header 340 from which the wall tubes 342 extend along the wall226 to the upper header 344. The latter is appropriately connected tothe drum 46. The lower headers 332 and 349 may be directly connected byfloor tubes 346.

Although the invention has been shown and described with reference tothe specific details of one embodiment, it is to be understood that theinvention is not to be taken as limited to all of the details shown anddescribed. It is rather to be taken as a scope commensurate with thescope of the subjoined claims.

What is claimed is:

1. In a high capacity steam generating and superheating unit of theradiant type, a furnace chamber having steam generating tubes along itswalls, fuel burning means for producing high temperature gases forpassage through the furnace chamber, a convection section including atubular steam heater receiving gases from the furnace chamber, meansnormally conducting steam to the steam heater, a division wall includingsteam generating tubes dividing the furnace chamber into two sectionswith portions of said tubes outlining a recycle gas outlet chamber orduct communicating with both sections of the furnace chamber at aposition remote from the fuel burning means, the division wall sodividing the furnace chamber that the sections of the latter are subjectto parallel flow of gases from the fuel burning means, the steam heaterhaving different parts exposed to the gas flows from the differentfurnace sections, and a recycled gas system including a fan with itsinlet communicating with the gas flow path downstream of the steamheating means and its outlet connected with said chamber or duct.

2. In a high capacity steam generating and superheating unit of theradiant type, a furnace chamber having steam generating tubes along itswalls, fuel burning means for producing high temperature gases forpassage through the furnace chamber, a convection section including asteam heater receiving gases from the furnace chamber, means normallyconducting steam to the steam heater, a division wall including steamgenerating tubes dividing the furnace chamber into two sections withoppositely bent out portions of said division tubes near the gas outletof the furnace chamber outlining a recycled gas outlet chamber or duct,the division wall so dividing the furnace chamber that the sections ofthe latter are subject to parallel flow of gases from the fuel burningmeans, the steam heater having different parts exposed to the gas flowsfrom the different furnace sections, and a recycled gas system includinga fan with its inlet connected with the gas flow path downstream of thesteam heating means and it's outlet connected with said chamber or duct.

3. A high capacity steam generating and superheating unit operating athigh steam pressures, walls including steam generating tubes defining afurnace chamber, a convection section including a convection steam,heater receiving furnace gases from the furnace chamber, means normallyconducting steam to the steam heater, fuel burning means producingcombustion gases for passage through the furnace chamber and thenthrough the con-, vection section, a division wall including steamgenerating tubes dividing the furnace chamber into two separate furnacechamber sections, the division wall so dividing the furnace chamber thatthe sections of the latter are subject to parallel flow of gases fromthe fuel burning means, the steam heater having dilferent parts exposedto the gas flows from the different furnace sections, some of saiddivision wall tubes being bent outwardly from one side of the divisionwall for a short vertical length and then returned to the wall alignmentbelow that position, others of the division wall tubes being bent in theopposite direction from said first bent division wall tubes and returning to the wall alignment at a similar lower position, said bentportions of the division wall tubes outlining a chamber or outlet ductextending into the furnace chamber sections from an opening in a wall ofthe furnace chamber section, and a recycled gas system including a fanand an inlet duct leading to the inlet of the fan from a position in thegas flow path downstream from the steam heating means, said recycled gassystem also including outlet duct means leading from the outlet of thefan to positions along the furnace chamber walls confronting saidrecycle gas outlet chamber formed by the bent division wall tubes and ata position remote from the fuel burning means, and means for varying theflow of recycled gases through said system to decrease said flow from amaximum at maximum load.

4. A high capacity steam generating and super-heating unit operating athigh steam pressures, walls including steam generating tubes outlining afurnace chamber, a convection section including a convection steamheater receiving furnace gases from the furnace chamber, fuel burningmeans producing combustion gases for passage through the furnace chamberand then through the convection section, means normally conducting steamto the steam heater, division wall including steam generating tubesdividing the furnace chamber section into two tubes dividing the furnacechamber section into two separate furnace chamber sections, the divisionwall so dividing the furnace chamber that the sections of the latter aresubject to parallel flow of gases from the fuel burning means, the steamheater having different parts exposed to the gas flows from thedifferent furnace sections, some of said division wall tubes being bentoutwardly from one side of the division wall for a short vertical lengthand then returned to the wall alignment below that position, said bentportions of some of the division wall tubes combining with otherdivision wall tubes to outline a chamber or outlet duct extending intothe furnace chamber sections from an opening in a wall of the furnacechamber, and a recycled gas system including a fan and an inlet ductleading to the inlet of the fan from a position in the gas flow pathdownstream from the steam heating means, said recycled gas system alsoincluding outlet duct means leading from the outlet of the fan topositions along the furnace chamber walls confronting said recycled gasoutlet chamber formed by the bent division wall tubes and at a positionremote from the fuel burning means, and means for varying the flow ofrecycled gases through said system to decrease said flow from a maximumat' maximum load.

5. In a high capacity steam generating and super-heating unit of theradiant type, a furnace chamber having steam generating tubes along itswalls, fuel burning means for producing high temperature gases forpassage through the furnace chamber, a convection section including asteam heater receiving gasesfrom the furnace chamber, means normallyconducting steam to the steam heater, a division wall including steamgenerating tubes dividing the furnace chamber into two sections withportions of said tubes remote from the fuel burning means defining arecycled gas duct, the division wall so dividing the furnace chamberthat the sections of the latter are subject 2,soo,1 1a

to parallel flow of gases from the fuel burning means, the steam heaterhaving different parts exposed to the gas flows from the differentfurnace sections, a recycled gas system including a fan with its inletconnected with the gas flow-path downstream of the steam heating meansand its outlet connected with said recycled gas duct, the recycled gasduct having openings for recycled gas flow into each furnace section,and means in each of the furnace walls opposite the division wallconnected with the recycled gas system outlet and having openingsthrough which recycled gas flows toward the division Wall.

6. A vapor generating and superheating unit having Walls defining afurnace chamber and a superjacent convection section arranged to receiveheating gases therefrom, vapor generating tubes lining walls of saidfurnace chamber, vapor superheating tubes arranged in said convectionsection, means providing a heating gas flow through said furnace chamberand convection section, vapor generating tubes arranged to define avertically extending division Wall dividing the furnace chamber into twoside-by-side sections having a parallel flow of heating gasestherethrough, a gas recirculation system including a fan with its inletcommunicating with said convection section downstream of said vaporsuperheating tubes, portions of said division wall tubes being bent todefine opposite sides of a gas outlet duct opening to both sections ofsaid furnace chamber, and conduit means connecting the outlet of saidfan to said gas outlet duct.

7. A vapor generating and superheating unit having walls defining afurnace chamber and a superjacent convection section arranged to receiveheating gases therefrom, vapor generating tubes lining Walls of saidfurnace chamber, vapor superheating tubes arranged in said convectionsection, means providing a heating gas flow through said furnace chamberand convection section, vapor generating tubes arranged to define avertically extending division wall dividing the furnace chamber into twoside-by-side sections having a parallel flow of heating gasestherethrough, a gas recirculation system including a fan with its inletcommunicating with said convection section downstream of said vaporsuperheating tubes, portions of said division Wall tubes being bent todefine opposite sides of a gas outlet duct opening to both sections ofsaid furnace chamber, conduit means connecting the outlet of said fan tosaid gas outlet duct, and means in each of the furnace chamber wallsopposite said division wall connected to said fan outlet and arranged todischarge recirculated gas towards said division wall.

References Cited in the file of this patent UNITED STATES PATENTS{BERTIFICAT OF CORRECTION Patent No. 2,800,116 Arthur E. Raynor et al.July 23, 1957 Y It is hereby certified that error appears .in theprinted specification of the above ni 'znbered patent requiringcorrection and that the said Letters Patent should 'rogad as correctedbelow.

001mm 8, line 38, after "heater," insert a---; line 40, strike out"tubes dividing the furnace chamber eection into two":

Signed and sealed this 17th day of June 1958.

(SEAL) Attest:

KARI- AXLINE ROBERT c. WATSON Atteeting Officer Conmissioner of Patents

